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How to identify Nylanderia fulva, the hairy crazy ant

Those of you paying attention to emerging pests probably know about the hairy crazy ant, Nylanderia sp. This little invader has been challenging fire ants as the primary nuisance ant along the U.S. gulf coast, first becoming dominant in Houston, Texas about 10 years ago and now expanding rapidly across the region. Crazy ants don’t sting, but their colonies have the habit of building up to disconcertingly large numbers of individuals. Some invaded sites are simply covered in ants (video!).

Writing about this insect can be difficult for nomenclatural reasons both vernacular and technical. The species variously goes by the names hairy crazy ant, caribbean crazy ant, and rasberry crazy ant, as well as an unsettled slate of scientific names: Nylanderia fulva, N. pubens, or N. sp. near fulva or pubens. Ongoing research by John LaPolla is currently sorting out the correct name. For reasons that I won’t delve into here, I call it Nylanderia fulva, the hairy crazy ant.

Nomenclature aside, our newly pesty North American form appears to be a single species. As this ant’s importance grows, so too does the importance of correct diagnosis. Thus, I have written this post as an informal tutorial to identifying these insects. A more rigorous account can be found at Joe MacGown’s site.

To use this guide, you’ll need a microscope powerful enough to count hairs on the back of a small ant.

Nylanderia are small ants, less than 4mm in length, with long, thick hairs arranged in rows on their backs.

Step 1. Verify that your ant belongs to the genus Nylanderia. These are small (2-4 mm) formicine ants, and their waists have one, rather than two, constricted segments. Within the formicines, Nylanderia is one of two genera with stiff, upright, bristly hairs arranged in rows on the back of the thorax. Many ants are hairy, but only Nylanderia (and Paratrechina– more on this below) sport their hairs in pairs or lines.

Nylanderia fulva/pubens has a dense coat of hair on the mesopleuron (Image: modified from Antweb.org)

Step 2. Verify that your ant is hairy enough. Most of the couple dozen Nylanderia species in North America are rather shiny in appearance. Nylanderia fulva, in comparison, is more dull as it carries a dense coat of fine hairs matted along much of its cuticle. In particular, only N. fulva and one other non-native species, Nylanderia bourbonica, show this coat along the mesopleuron (the side of the middle of the thorax) as indicated in the figure above.

Nylanderia flavipes is another non-native Nylanderia, but like most species its flanks are shining (Image: modified from Antweb.org)

The black crazy ant Paratrechina longicornis has similarly long hairs to Nylanderia, but it is much more slender. It also lacks hairs on the mesopleuron. (Image: modified from Antweb.org)

If your ant is hairless or only sparsely hairy on the side of the thorax, it’s not a hairy crazy ant.

Nylanderia bourbonica is also unusually hairy for the genus, but it is much darker in color than the hairy crazy ant, and the sides of its thorax usually have a somewhat sparser coating of hairs. (Image: modified from Antweb.org)

Step 3.Check the color: Reddish-brown, or brownish-black? Nylanderia bourbonica– not the hairy crazy ant, but often found in similar places- is darker in color. If you’ve got hairy crazy ants, they will be a light to medium reddish brown.

In the field, hairy crazy ants are reddish-brown and rather pudgy.

There you have it. If your mystery ant is in the southern United States, is reddish-brown, is 3-4mm long, bears rows of stiff hairs on its back, and has a dense coat of fine hairs on its sides, you’ve likely got Nylanderia fulva.

Late to the game here, but I finally sat down and thought through the paper. I definitely agree that the paramere character is a keeper, but I would also like to point out Gotzek et al.’s point “It appears at this time that N. pubens is restricted
to the Caribbean region.” So, as an emendation to Alex’s key:

Step 4: ask yourself “am I in the Caribbean or the U.S. mainland?”
-> U.S. mainland: N. fulva
-> Caribbean: refer to males or step 6.

Nicely done as always Alex. As a researcher of Nylanderia sp. in Texas, I contend that there are morphological differences between the Texas and Florida N. sp. nr. pubens, as described by Meyers and Gold (2008)

Based on samples that we have seen from Florida, there are always 3-4 pairs of macrosetae on the mesonotum of workers. Whereas, there are almost always only 2 pairs (very rarely 3) in the Texas populations. This may not constitute a seperate species, but as you mentioned, LaPolla’s research should clear this up for us. This recent publication by Zhao et. al. suggests that the two populations are the same species:

Now that you’ve identified them, can you get them produce sterile offspring? We’re in the middle of a plague down here! We can’t garden, run electric fencing for livestock or keep our rentals rented. HELP!

It looks like the crazy ants ate the author and commenters since there has been no replies for years… Hey… What’s that strange noise… A soft tapping… Like rain… OH NO! They’re here! They’ve come to eat me! Help me! Help me! AAARRRGGGHHHhhh….